Fix install tree root length

[michaelmckinsey1]

Description

• isolate bootstrap cache to workspace spack instance #893 made a change that causes all of the gitlab tests to fail, because the name bootstrap-cache made the install tree root too long for spack/bin/sbang. This is a temporary fix to change the folder name to b-c (limited to three characters) until Fix bootstrap-cache name by configuring CI path #913 is resolved.
• Tracking issue in Fix bootstrap-cache name by configuring CI path #913
• See failure https://lc.llnl.gov/gitlab/benchpark/benchpark/-/jobs/2771016

Adding/modifying core functionality, CI, or documentation:

• Update lib/benchpark/runtime.py

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 41.27%. Comparing base (1339b16) to head (7a8ce21).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #911   +/-   ##
========================================
  Coverage    41.27%   41.27%           
========================================
  Files           22       22           
  Lines         1519     1519           
  Branches       215      215           
========================================
  Hits           627      627           
  Misses         847      847           
  Partials        45       45           

Flag	Coverage Δ
unittests	41.27% <ø> (ø)

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